Solidly-mounted transversely-excited film bulk acoustic resonator with recessed interdigital transducer fingers

ABSTRACT

Acoustic resonator devices, filters, and methods are disclosed. An acoustic resonator includes a substrate, a piezoelectric plate having front and back surfaces, and an acoustic Bragg reflector between a surface of the substrate and the back surface of the piezoelectric plate. An interdigital transducer (IDT) is formed on the front surface of the piezoelectric plate. The IDT is configured to excite a shear primary acoustic mode in the piezoelectric plate in response to a radio frequency signal applied to the IDT. All fingers of the IDT are disposed in a respective grooves in the piezoelectric plate.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 16/995,482, titledSOLIDLY-MOUNTED TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITHRECESSED INTERDIGITAL TRANSDUCER FINGERS, filed Aug. 17, 2020, which isa continuation of application Ser. No. 16/518,594, titled TRANSVERSELYEXCITED FILM BULK ACOUSTIC RESONATOR USING ROTATED Z-CUT LITHIUMNIOBATE, filed Jul. 22, 2019, now U.S. Pat. No. 10,797,675.

Application Ser. No. 16/518,594 claims priority to provisionalapplication 62/842,161, titled XBAR RESONATORS ON ROTATED Z-CUT LITHIUMNIOBATE, filed May 2, 2019. Application Ser. No. 16/518,594 is also acontinuation-in-part of application Ser. No. 16/230,443, filed Dec. 21,2018, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S.Pat. No. 10,491,192, which claims priority from the followingprovisional applications: application 62/685,825, filed Jun. 15, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20,2018, entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filedOct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR(XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODEFILM BULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. Application Ser. No. 16/518,594 is also acontinuation-in-part of application Ser. No. 16/438,141, filed Jun. 11,2019, now U.S. Pat. No. 10,601,392 titled SOLIDLY MOUNTEDTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, which is acontinuation-in-part of application Ser. No. 16/230,443, now U.S. Pat.No. 10,491,192. Application Ser. No. 16/438,141 also claims priorityfrom provisional patent application 62/753,809, filed Oct. 31, 2018,titled SOLIDLY MOUNTED SHEAR-MODE FILM BULK ACOUSTIC RESONATOR, andprovisional patent application 62/818,564, filed Mar. 14, 2019, titledSOLIDLY MOUNTED XBAR. All of these applications are incorporated hereinby reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to filters, oscillators, sensors and other radiofrequency devices using acoustic wave resonators, and specifically tofilters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative expanded schematic cross-sectional view of anXBAR.

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 5 is a graphical representation of Euler angles.

FIG. 6 is a chart of the electromechanical coupling of an XBAR as afunction of Z-axis tilt angle β.

FIG. 7 is a chart of the resonance and anti-resonance frequencies of anXBAR as a function of Z-axis tilt angle β.

FIG. 8 is a chart of the Q factor at the resonance and anti-resonancefrequencies of an XBAR as a function of Z-axis tilt angle β.

FIG. 9 is a chart of the admittance of two XBAR devices as functions offrequency.

FIG. 10 is a schematic circuit diagram of a filter using XBARs.

FIG. 11 is a flow chart of a process for fabricating an XBAR.

FIG. 12 is a flow chart of a process for fabricating a solidly mountedXBAR.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously described element having the same reference designator.

DETAILED DESCRIPTION Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely excited film bulk acousticresonator (XBAR) 100. XBARs were first described in application Ser. No.16/230,443. XBAR resonators such as the resonator 100 may be used in avariety of RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are particularly suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of lithium niobate. The piezoelectric plate is cutsuch that the orientation of the x, y, and z crystalline axes withrespect to the front and back surfaces is known and consistent. Inparticular, the piezoelectric plate 110 is rotated z-cut, which is tosay the z crystalline axis is inclined by a small rotation anglerelative to the normal to the front and back surfaces 112, 114. As willbe discussed in further detail, the small rotation angle is defined bythe second Euler angle of the piezoelectric plate.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thecavity 140 is a hole though the substrate 110. In other configurations,the cavity 140 may be a recess in the substrate 120. Also as shown inFIG. 1, the diaphragm 115 is contiguous with the rest of thepiezoelectric plate 110 around all of a perimeter 145 of the cavity 140.In this context, “contiguous” means “continuously connected without anyintervening item”. In other configurations, there may be openingsthrough the piezoelectric plate 110 (for example to allow etching of thecavity beneath the piezoelectric plate). In this case, the diaphragm 115will be contiguous with the rest of the piezoelectric plate 110 aroundat least 50% of the perimeter 145 of the cavity.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120. The cavity 140 may be formed, for example, by selective etching ofthe substrate 120 before or after the piezoelectric plate 110 and thesubstrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates back and forth along a direction substantially orthogonal tothe surface of the piezoelectric plate 110, which is also normal, ortransverse, to the primary direction of the electric field createdbetween the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of Lithium Niobatepiezoelectrical material having parallel front and back surfaces 112,114 and a thickness ts. ts may be, for example, 100 nm to 1500 nm. Whenused in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g. LTE™ bands42, 43, 46), the thickness ts may be, for example, 300 nm to 700 nm.

The IDT fingers, such as IDT finger 236, may be disposed on the frontsurface 112 of the piezoelectric plate 110. Alternatively, IDT fingers,such as IDT finger 238, may be disposed in grooves formed in the frontsurface 112. The IDT fingers 236, 238 may be aluminum, substantiallyaluminum alloys, copper, substantially copper alloys, beryllium, gold,tungsten, molybdenum or some other conductive material. Thin (relativeto the total thickness of the conductors) layers of other metals, suchas chromium or titanium, may be formed under and/or over the fingers toimprove adhesion between the fingers and the piezoelectric plate 110and/or to passivate or encapsulate the fingers. The busbars (132, 134 inFIG. 1) of the IDT may be made of the same or different materials as thefingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the wavelength of the horizontally-propagatingsurface acoustic wave at the resonance frequency. Additionally, themark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5(i.e. the mark or finger width is about one-fourth of the acousticwavelength at resonance). In an XBAR, the pitch p of the IDT istypically 2 to 20 times the width w of the fingers. In addition, thepitch p of the IDT is typically 2 to 20 times the thickness ts of thepiezoelectric plate 112. The width of the IDT fingers in an XBAR is notconstrained to one-fourth of the acoustic wavelength at resonance. Forexample, the width of XBAR IDT fingers may be 500 nm or greater, suchthat the IDT can be fabricated using optical lithography.

The thickness tm of the IDT fingers may be from 100 nm to about equal tothe width w. The thickness of the busbars (132, 134 in FIG. 1) of theIDT may be the same as, or greater than, the thickness tm of the IDTfingers. The depth tg of the grooves formed in the front surface may beless than (as shown in FIG. 2), equal to, or greater than tm.

A front-side dielectric layer 214 may optionally be formed on the frontsurface 112 of the piezoelectric plate 110. The “front side” of the XBARis, by definition, the surface facing away from the substrate. Thefront-side dielectric layer 214 has a thickness tfd. The front-sidedielectric layer 214 is formed between the IDT fingers 236, 238.Although not shown in FIG. 2, the front side dielectric layer 214 mayalso be deposited over the IDT fingers 238. The front-side dielectriclayer 214 may be a non-piezoelectric dielectric material, such assilicon dioxide or silicon nitride. tfd may be, for example, 0 to 500nm. tfd is typically less than the thickness ts of the piezoelectricplate. The front-side dielectric layer 214 may be formed of multiplelayers of two or more materials. The front-side dielectric layer may bedeposited, for example by evaporation, sputtering, chemical vapordeposition, or some other technique.

FIG. 3 shows a detailed schematic cross-sectional view of a solidlymounted XBAR (SM XBAR) 300. SM XBARs were first described in applicationSer. No. 16/381,141. The SM XBAR 300 includes a piezoelectric plate 110,an IDT (of which only fingers 236 are visible) and an optionalfront-side dielectric layer 214 as previously described. Thepiezoelectric layer 110 has parallel front and back surfaces 112, 114.Dimension is is the thickness of the piezoelectric plate 110. The widthof the IDT fingers 236 is dimension w, thickness of the IDT fingers isdimension tm, and the IDT pitch is dimension p. The thickness of thefront-side dielectric layer 214 is dimension tfd.

In contrast to the XBAR devices shown in FIG. 1 and FIG. 2, the IDT ofan SM XBAR is not formed on a diaphragm spanning a cavity in thesubstrate 120. Instead, an acoustic Bragg reflector 340 is sandwichedbetween a surface 222 of the substrate 220 and the back surface 114 ofthe piezoelectric plate 110. The term “sandwiched” means the acousticBragg reflector 340 is both disposed between and mechanically attachedto a surface 222 of the substrate 220 and the back surface 114 of thepiezoelectric plate 110. In some circumstances, thin layers ofadditional materials may be disposed between the acoustic Braggreflector 340 and the surface 222 of the substrate 220 and/or betweenthe Bragg reflector 340 and the back surface 114 of the piezoelectricplate 110. Such additional material layers may be present, for example,to facilitate bonding the piezoelectric plate 110, the acoustic Braggreflector 340, and the substrate 220.

The acoustic Bragg reflector 340 includes multiple dielectric layersthat alternate between materials having high acoustic impedance andmaterials have low acoustic impedance. “High” and “low” are relativeterms. For each layer, the standard for comparison is the adjacentlayers. Each “high” acoustic impedance layer has an acoustic impedancehigher than that of both the adjacent low acoustic impedance layers.Each “low” acoustic impedance layer has an acoustic impedance lower thanthat of both the adjacent high acoustic impedance layers. As will bediscussed subsequently, the primary acoustic mode in the piezoelectricplate of an XBAR is a shear bulk wave. Each of the layers of theacoustic Bragg reflector 340 has a thickness equal to, or about,one-fourth of the wavelength of a shear bulk wave having the samepolarization as the primary acoustic mode at or near a resonancefrequency of the SM XBAR 300. Dielectric materials having comparativelylow acoustic impedance include silicon dioxide, carbon-containingsilicon oxide, and certain plastics such as cross-linked polyphenylenepolymers. Materials having comparatively high acoustic impedance includehafnium oxide, silicon nitride, aluminum nitride, silicon carbide. Allof the high acoustic impedance layers of the acoustic Bragg reflector340 are not necessarily the same material, and all of the low acousticimpedance layers are not necessarily the same material. In the exampleof FIG. 3, the acoustic Bragg reflector 340 has a total of six layers.An acoustic Bragg reflector may have more than, or less than, sixlayers.

The IDT fingers, such as IDT finger 336, may be disposed on the frontsurface 112 of the piezoelectric plate 110. Alternatively, IDT fingers,such as IDT finger 338, may be disposed in grooves formed in the frontsurface 112. The grooves may extend partially through the piezoelectricplate, as shown in FIG. 2. Alternatively, the grooves may extendcompletely through the piezoelectric plate as shown in FIG. 3.

FIG. 4 is a graphical illustration of the primary acoustic mode in anXBAR. FIG. 4 shows a small portion of an XBAR 400 including apiezoelectric plate 410 and three interleaved IDT fingers 430. An RFvoltage is applied to the interleaved fingers 430. This voltage createsa time-varying electric field between the fingers. The direction of theelectric field is predominantly lateral, or parallel to the surface ofthe piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric energy is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites a shear-mode acoustic mode, inthe piezoelectric plate 410. In this context, “shear deformation” isdefined as deformation in which atomic displacements are horizontal butvary in a vertical direction. A “shear acoustic mode” is defined as anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of physical motion of thepiezoelectric media. The degree of physical motion, as well as thethickness of the piezoelectric plate 410, have been greatly exaggeratedfor ease of visualization. While the atomic motions are predominantlylateral (i.e. horizontal as shown in FIG. 4), the direction of acousticenergy flow of the excited primary shear acoustic mode is substantiallyorthogonal to the surface of the piezoelectric plate, as indicated bythe arrow 465.

In an SM XBAR, as shown in FIG. 3, the motion distribution in thepiezoelectric plate is similar. However the thickness of the plate isnot necessarily close to one-half of the wavelength of the primaryacoustic mode, and some part of acoustic energy is localized in theBragg stack, in shear vibrations with amplitude exponentially decayingin the depth of the stack.

Considering FIG. 4, there is essentially no horizontal electric fieldimmediately under the IDT fingers 430, and thus acoustic modes are onlyminimally excited in the regions 470 under the fingers. There may beevanescent acoustic motions in these regions. Since acoustic vibrationsare not excited under the IDT fingers 430, the acoustic energy coupledto the metal IDT fingers 430 is low (for example compared to the fingersof an IDT in a SAW resonator), which minimizes viscous losses in the IDTfingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. This compression/extension of theelastic media is responsible for additional adiabatic loss mechanismabsent for pure shear waves. In addition, the strongest coupling inlithium niobate and lithium tantalate corresponds to the sheardeformations. Thus, the piezoelectric coupling for shear wave XBARresonances can be high (>20%) compared to other acoustic resonators.High piezoelectric coupling enables the design and implementation ofmicrowave and millimeter-wave filters with appreciable bandwidth.

FIG. 5 is a graphical illustration of Euler angles. Euler angles are asystem, introduced by Swiss mathematician Leonhard Euler, to define theorientation of a body with respect to a fixed coordinate system. Theorientation is defined by three successive rotations about angles α, β,and γ.

As applied to acoustic wave devices, xyz is a three-dimensionalcoordinate system aligned with the crystalline axes of the piezoelectricmaterial. XYZ is a three-dimensional coordinate system aligned with theacoustic wave device, where Z is normal to the surface of thepiezoelectric material. XY is the plane of the surface of thepiezoelectric material. X is the direction of the electric field andacoustic wave propagation for SAW and most plate-mode devices, and Y istypically parallel to the fingers on an IDT. In XBAR devices, X is thedirection of the electric field, but acoustic wave propagation isdominantly along the Z direction. All of the XBAR devices described inapplication Ser. No. 16/230,443 and application Ser. No. 16/381,141 usepiezoelectric plates with the z axis normal to the plate surface and they axis orthogonal to the IDT fingers. Such piezoelectric plates haveEuler angles of 0, 0, 90°.

FIG. 6 is a chart 600 of the electromechanical coupling coefficient ofrepresentative XBAR devices using piezoelectric plates with Euler angles0, β, 90°, where β is in the range from −15° to +5°. The charts in FIG.6, FIG. 7, FIG. 8, and FIG. 9 are based on simulation of XBAR devicesusing finite element methods. The line 610 is a plot ofelectromechanical coupling coefficient as a function of β. Inspection ofthe chart 600 shows that the electromechanical coupling coefficient isgreater than 0.26 for β greater than or equal to −11° and less than orequal to −5°, as compared to a value of about 0.243 for β=0.

An increase in electromechanical coupling coefficient by 0.017effectively increases the frequency difference between the resonance andanti-resonance frequencies of an XBAR, as shown in FIG. 7. FIG. 7 is achart 700 of the resonance and anti-resonance frequencies ofrepresentative XBAR devices using piezoelectric plates with Euler angles0, β, 90°. The solid line 710 is a plot of anti-resonance frequency as afunction of β. The dashed line 720 is a plot of resonance frequency as afunction of β. Inspection of the chart 700 shows that the differencebetween the anti-resonance and resonance frequencies is about 655 MHzfor β greater than or equal to −10° and less than or equal to −7.5°, ascompared to 604 MHz for β=0. A 50 MHz, of about 8%, increase in thedifference between the anti-resonance and resonance frequencies of aresonator facilitates the design of broad bandwidth band-pass filters.

FIG. 8 is a chart 800 of the Q-factor at the resonance andanti-resonance frequencies of representative XBAR devices usingpiezoelectric plates with Euler angles 0, β, 90°. In determiningQ-factor, only material losses (i.e. acoustic absorption) in thepiezoelectric plate and electrodes are considered. Resistive losses inthe electrodes and side and edge radiation effects, all of which arenearly independent of β, are ignored. The line 810 is a plot of Q-factorat the anti-resonance frequency as a function of β. The line 820 is aplot of Q-factor at the resonance frequency as a function of β.Inspection of the chart 800 shows that the Q-factor at theanti-resonance frequency is relatively independent of β, and theQ-factor at the resonance frequency is maximum for β=about −6°. TheQ-factor at the resonance frequency is above 3000 for β greater than orequal to −9° and less than or equal to −2.5°, as compared to a Q-factorof about 2700 for β=0. An increase in the Q-factor at the resonancefrequency may allow the development of band-pass filters with a sharpertransition at the lower edge of the passband and/or lower insertionloss.

The improvement in Q-factor at the resonance frequency for β greaterthan or equal to −9° and less than or equal to −2.5° can be explained bycoincidence of the direction of the group and phase velocities at aboutthese angles. For β=0, the group velocity has non-zero x-component,corresponding to the slight drift of energy outside the resonator.

FIG. 9 is a chart 900 showing the normalized magnitude of the admittance(on a logarithmic scale) as a function of frequency for two XBAR devicessimulated using finite element method (FEM) simulation techniques. Thedashed line 920 is a plot of the admittance on an XBAR on a Z-cutlithium niobate plate. In this case the Z crystalline axis is orthogonalto the surfaces of the plate and the Euler angles are 0, 0, 90°. Thesolid line 910 is a plot of the admittance of an XBAR on a −7.5° rotatedZ-cut lithium niobate plate. In this case, the Z crystalline axis isinclined by +7.5° with respect to orthogonal to the surfaces of theplate and the Euler angles are 0, −7.5°, 90°. The IDT fingers arealuminum. The IDT is oriented such that the crystalline y-axis of thepiezoelectric plate is normal to the IDT fingers. The substratesupporting the piezoelectric plate is silicon with a cavity formed underthe IDT fingers. The simulated physical dimensions are as follows:ts=400 nm; tfd=0; tm=300 nm; p=5 um; w=300 nm, AP=100 um, L=1 mm.

The difference between anti-resonance and resonance frequencies of theresonator on the rotated Z-cut plate (solid line 910) is significantlygreater than the difference between anti-resonance and resonancefrequencies of the resonator on the Z-cut plate (dashed line 920). Thisis indicative of the larger electromechanical coupling available withthe rotated Z-cut plate.

FIG. 10 is a schematic circuit diagram for an exemplary embodiment of ahigh frequency band-pass filter 1000 using XBARs. The filter 1000 has aconventional ladder filter architecture including three seriesresonators 1010A, 1010B, 1010C and two shunt resonators 1020A, 1020B.The three series resonators 1010A, 1010B, and 1010C are connected inseries between a first port and a second port. In FIG. 10, the first andsecond ports are labeled “In” and “Out”, respectively. However, thefilter 1000 is symmetrical and either port and serve as the input oroutput of the filter. The two shunt resonators 1020A, 1020B areconnected from nodes between the series resonators to ground. All theshunt resonators and series resonators are XBARs or SM XBARs.

The three series resonators 1010A, B, C and the two shunt resonators1020A, B of the filter 1000 may be formed on a single plate 1030 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown). When the resonators areXBARs, at least the fingers of each IDT are disposed over a cavity inthe substrate. In FIG. 10, the cavities are illustrated schematically asthe dashed rectangles (such as the rectangle 1035). In this example,each IDT is disposed over a respective cavity. In this and similarcontexts, the term “respective” means “relating things each to each”,which is to say with a one-to-one correspondence. In other filters, theIDTs of two or more resonators may be disposed over a single cavity.When the resonators are SM XBARs, the IDTs are disposed over an acousticBragg reflector as shown in FIG. 3.

Description of Methods

FIG. 11 is a simplified flow chart showing a process 1100 for making anXBAR or a filter incorporating XBARs. The process 1100 starts at 1105with a substrate and a plate of piezoelectric material and ends at 1195with a completed XBAR or filter. The flow chart of FIG. 11 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 11.

The flow chart of FIG. 11 captures three variations of the process 1100for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 1110A, 1110B, or1110C. Only one of these steps is performed in each of the threevariations of the process 1100.

The piezoelectric plate may be, for example, rotated Z-cut lithiumniobate. The Euler angles of the piezoelectric plate are 0, β, 90°,where β is in the range from −15° to +5°. Preferably, β may be in therange from −11° to −5° to maximize electromechanical coupling. β may bein the range from −10° to −7.5° to maximize Q-factor at the resonancefrequency. The substrate may preferably be silicon. The substrate may besome other material that allows formation of deep cavities by etching orother processing.

In one variation of the process 1100, one or more cavities are formed inthe substrate at 1110A, before the piezoelectric plate is bonded to thesubstrate at 1120. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1110A will not penetrate through the substrate.

At 1120, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1130 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1130 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1130 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 1140, a front-side dielectric layer may be formed by depositing oneor more layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 1100, one or more cavities areformed in the back side of the substrate at 1110B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 1100, a back-side dielectriclayer may be formed at 1150. In the case where the cavities are formedat 1110B as holes through the substrate, the back-side dielectric layermay be deposited through the cavities using a conventional depositiontechnique such as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 1100, one or more cavities in theform of recesses in the substrate may be formed at 1110C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 1100, the filter device is completed at1160. Actions that may occur at 1160 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at1160 is to tune the resonant frequencies of the resonators within thedevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 1195.

FIG. 12 is a simplified flow chart of a method 1200 for making a SM XBARor a filter incorporating SM XBARs. The method 1200 starts at 1210 witha thin piezoelectric plate disposed on a sacrificial substrate 1202 anda device substrate 1204. The method 1200 ends at 1295 with a completedSM XBAR or filter. The flow chart of FIG. 12 includes only major processsteps. Various conventional process steps (e.g. surface preparation,cleaning, inspection, baking, annealing, monitoring, testing, etc.) maybe performed before, between, after, and during the steps shown in FIG.12.

The piezoelectric plate 1202 may be, for example, rotated Z-cut lithiumniobate. The Euler angles of the piezoelectric plate are 0, β, 90°,where β is in the range from −15° to +5°. Preferably, β may be in therange from −11° to −5° to maximize electromechanical coupling. β may bein the range from −10° to −7.5° to maximize Q-factor at the resonancefrequency.

At 1220, an acoustic Bragg reflector is formed by depositing alternatingdielectric layers of high acoustic impedance and low acoustic impedancematerials. Each of the layers has a thickness equal to or aboutone-fourth of the acoustic wavelength. Dielectric materials havingcomparatively low acoustic impedance include silicon dioxide, siliconoxycarbide, and certain plastics such as cross-linked polyphenylenepolymers. Dielectric materials having comparatively high acousticimpedance include silicon nitride and aluminum nitride. All of the highacoustic impedance layers are not necessarily the same material, and allof the low acoustic impedance layers are not necessarily the samematerial. The total number of layers in the acoustic Bragg reflector maybe from about five to more than twenty.

At 1220, all of the layers of the acoustic Bragg reflector may bedeposited on either the surface of the piezoelectric plate on thesacrificial substrate 1202 or a surface of the device substrate 1204.Alternatively, some of the layers of the acoustic Bragg reflector may bedeposited on the surface of the piezoelectric plate on the sacrificialsubstrate 1202 and the remaining layers of the acoustic Bragg reflectormay be deposited on a surface of the device substrate 1204.

At 1230, the piezoelectric plate on the sacrificial substrate 1202 andthe device substrate 1204 may be bonded such that the layers of theacoustic Bragg reflector are sandwiched between the piezoelectric plateand the device substrate. The piezoelectric plate on the sacrificialsubstrate 1202 and the device substrate 1204 may be bonded using a waferbonding process such as direct bonding, surface-activated orplasma-activated bonding, electrostatic bonding, or some other bondingtechnique. Note that, when one or more layers of the acoustic Braggreflector are deposited on both the piezoelectric plate and the devicesubstrate, the bonding will occur between or within layers of theacoustic Bragg reflector.

After the piezoelectric plate on the sacrificial substrate 1202 and thedevice substrate 1204 may be bonded, the sacrificial substrate, and anyintervening layers, are removed at 1240 to expose the surface of thepiezoelectric plate (the surface that previously faced the sacrificialsubstrate).The sacrificial substrate may be removed, for example, bymaterial-dependent wet or dry etching or some other process.

At least one groove can be formed in the piezoelectric plate at step1245. Grooves may be formed using conventional techniques such asetching through a patterned photoresist layer. The grooves can be sizedand shaped to accommodate portions of a conductor pattern, such asfingers of an IDT.

A conductor pattern, including IDTs of each SM XBAR, is formed at 1250by depositing and patterning one or more conductor layer on the surfaceof the piezoelectric plate that was exposed when the sacrificialsubstrate was removed at 1240. At least one finger of an IDT can be in agroove in the piezoelectric plate formed at 1245. The conductor patternmay be, for example, aluminum, an aluminum alloy, copper, a copperalloy, or some other conductive metal. Optionally, one or more layers ofother materials may be disposed below (i.e. between the conductor layerand the piezoelectric plate) and/or on top of the conductor layer. Forexample, a thin film of titanium, chrome, or other metal may be used toimprove the adhesion between the conductor layer and the piezoelectricplate. A conduction enhancement layer of gold, aluminum, copper or otherhigher conductivity metal may be formed over portions of the conductorpattern (for example the IDT bus bars and interconnections between theIDTs).

The conductor pattern may be formed at 1250 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1250 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 1260, one or more optional front-side dielectric layers may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. The one or more dielectric layers maybe deposited using a conventional deposition technique such assputtering, evaporation, or chemical vapor deposition. The one or moredielectric layers may be deposited over the entire surface of thepiezoelectric plate, including on top of the conductor pattern.Alternatively, one or more lithography processes (using photomasks) maybe used to limit the deposition of the dielectric layers to selectedareas of the piezoelectric plate, such as only between the interleavedfingers of the IDTs. Masks may also be used to allow deposition ofdifferent thicknesses of dielectric materials on different portions ofthe piezoelectric plate. For example, a first dielectric layer having afirst thickness t1 may be deposited over the IDTs of one or more shuntresonators. A second dielectric layer having a second thickness t2,where t2 is equal to or greater than zero and less than t1, may bedeposited over the IDTs of series resonators.

After the conductor pattern and optional front-side dielectric layer areformed at 1250 and 1260, the filter device may be completed at 1270.Actions that may occur at 1270 including depositing and patterningadditional metal layers to form conductors other than the IDT conductorpattern; depositing an encapsulation/passivation layer such as SiO₂ orSi₃O₄ over all or a portion of the device; forming bonding pads orsolder bumps or other means for making connection between the device andexternal circuitry; excising individual devices from a wafer containingmultiple devices; other packaging steps; and testing. Another actionthat may occur at 1270 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 1295.

A variation of the process 1200 starts with a single-crystalpiezoelectric wafer at 1202 instead of a thin piezoelectric plate on asacrificial substrate of a different material. Ions are implanted to acontrolled depth beneath a surface of the piezoelectric wafer (not shownin FIG. 12). The portion of the wafer from the surface to the depth ofthe ion implantation is (or will become) the thin piezoelectric plateand the balance of the wafer is the sacrificial substrate. The acousticBragg reflector is formed at 1220 as previously described and thepiezoelectric wafer and device substrate are bonded at 1230 such thatthe acoustic Bragg reflector is disposed between the ion-implantedsurface of the piezoelectric wafer 1202 and the device substrate 1204.At 1240, the piezoelectric wafer may be split at the plane of theimplanted ions (for example, using thermal shock), leaving a thin plateof piezoelectric material exposed and bonded to the acoustic Braggreflector. The thickness of the thin plate piezoelectric material isdetermined by the energy (and thus depth) of the implanted ions. Theprocess of ion implantation and subsequent separation of a thin plate iscommonly referred to as “ion slicing”.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a piezoelectric plate having front and back surfaces;an acoustic Bragg reflector between the surface of the substrate and theback surface of the piezoelectric plate; and an interdigital transducer(IDT) formed on the front surface of the piezoelectric plate, the IDTand the piezoelectric plate configured such that a radio frequencysignal applied to the IDT excites a shear primary acoustic mode in thepiezoelectric plate, wherein all fingers of the IDT are disposed inrespective grooves in the piezoelectric plate.
 2. The device of claim 1,wherein a direction of acoustic energy flow of the shear primaryacoustic mode is substantially orthogonal to the front and back surfacesof the piezoelectric plate.
 3. The device of claim 1, wherein a depth ofthe grooves is less than or equal to a thickness of the piezoelectricplate.
 4. The device of claim 1, wherein a depth of the grooves is lessthan a thickness of the finger.
 5. The device of claim 1, wherein adepth of the grooves is equal to a thickness of the finger.
 6. Thedevice of claim 1, wherein the piezoelectric plate is a rotated z-cutlithium niobate plate.
 7. A filter device, comprising: a substrate; apiezoelectric plate having front and back surfaces; an acoustic Braggreflector between the surface of the substrate and the back surface ofthe piezoelectric plate; and a conductor pattern formed on the frontsurface, the conductor pattern including a plurality of interdigitaltransducers (IDTs) of a respective plurality of acoustic resonators,wherein all of the IDTs are configured to excite respective shearprimary acoustic modes in the piezoelectric plate in response torespective radio frequency signals applied to each IDT, wherein allfingers of at least one of the plurality of IDTs are disposed inrespective grooves in the piezoelectric plate.
 8. The filter device ofclaim 7, wherein directions of acoustic energy flow of all of the shearprimary acoustic modes are substantially orthogonal to the front andback surfaces of the piezoelectric plate.
 9. The filter device of claim7, wherein a depth of the grooves is less than or equal to a thicknessof the piezoelectric plate.
 10. The filter device of claim 7, wherein adepth of the grooves is less than a thickness of the finger.
 11. Thefilter device of claim 7, wherein a depth of the grooves is equal to athickness of the finger.
 12. The filter device of claim 7, wherein thepiezoelectric plate is a rotated z-cut lithium niobate plate.
 13. Amethod of fabricating an acoustic resonator device, comprising: formingan acoustic Bragg reflector by depositing material layers on one or bothof a surface of a device substrate and a back surface of a piezoelectricplate having a front surface attached to a sacrificial substrate;bonding the piezoelectric plate to the device substrate such that layersof the acoustic Bragg reflector are between the back surface of thepiezoelectric plate and the surface of the device substrate; removingthe sacrificial substrate to expose the front surface of thepiezoelectric plate; and forming an interdigital transducer (IDT) on thefront surface of the piezoelectric plate such that interleaved fingersof the IDT are configured to excite a shear primary acoustic mode in thepiezoelectric plate in response to a radio frequency signal applied tothe IDT, wherein all fingers of the IDT are disposed in respectivegrooves in the piezoelectric plate.
 14. The method of claim 13, whereina depth of the grooves is less than or equal to a thickness of thepiezoelectric plate.
 15. The method of claim 13, wherein a depth of thegrooves is less than a thickness of the fingers of the IDT.
 16. Themethod of claim 13, wherein a depth of the grooves is equal to athickness of the fingers of the IDT.
 17. The method of claim 13, whereinthe piezoelectric plate is a rotated z-cut lithium niobate plate.